A material forming a substrate for a semiconductor device must have a thermal expansion coefficient which is substantially similar to that of the semiconductor device, so that no distortion is caused by thermal stress. Thus, a material having a small difference in its thermal expansion coefficient, as compared to the semiconductor device, has been generally made of an Ni alloy such as ASTM F-15 (29 wt.% Ni - 17 wt.% Co--Fe) or ASTM F-30 (42 wt. % Ni--Fe), or a ceramic material such as alumina or forsterite.
In recent years, however, the semiconductor technique has been remarkably improved, resulting in an increase in scale and circuit density of such semiconductor devices. However, a problem of proper heat dissipation has arisen in addition to the aforementioned difference in thermal expansion coefficient. In other words, not only a small difference between the thermal expansion coefficients is needed, an excellent heat dissipation is also required as the semiconductor devices increase in size and circuit density.
Under such circumstances, beryllia, tungsten, molybdenum or the like has been generally proposed as a material which satisfies both of the above requirements.
However, beryllia, which is toxic, is practically unemployable in consideration of a health hazard and environmental pollution.
On the other hand, molybdenum and tungsten are very expensive since these elements are produced in few areas of the world in small amounts. Thus, use of such metal materials leads to increased costs for the semiconductor devices. Besides, such metals are heavy because of a relatively high density (19.3 g/cm.sup.3 for tungsten and 10.2 g/cm.sup.3 for molybdenum), while the same are relatively hard to machine.
Thus, a lightweight substrate material for a semiconductor device, which can satisfy both of the required characteristics of excellent heat dissipation and a small difference in thermal expansion coefficients while also having an excellent machinability, is very desirable.
Japanese Patent Publication No. 63-16458(1988) discloses an aluminum alloy prepared by adding 30 to 50 percent by weight of silicon to the aluminum. Such an alloy is used as a substrate material for a semiconductor device.
However, although the aforementioned alloy of Al - 30 to 50 wt. % Si has a thermal expansion coefficient which is substantially similar to that of a semiconductor element, its heat dissipation is reduced as compared with that of an ordinary aluminum alloy containing a small amount of silicon. In general, it has been possible to cope with the heat generation in a semiconductor device even if the heat dissipation of the aluminum alloy is reduced. As to a higher heating value following a high integration of a recent semiconductor element, however, it is difficult to sufficiently dissipate heat generated in the semiconductor element by a radiation member formed of only the aforementioned Al--Si alloy.
On the other hand, lightness and a high reliability against moisture resistance, heat resistance, etc. are particularly required for a package employed for a small and lightweight microwave integrated circuit device which is suitable for a mobile radio station such as a communication satellite, an aircraft or the like. In general, the package for a microwave integrated circuit device is made of a ferrous material such as stainless steel, iron-nickel-42% alloy or the like. However, such a ferrous material has a relatively large specific gravity of about 8 g/cm.sup.3. Therefore, the ferrous material has been unsatisfactory as a package material for a microwave integrated circuit device which is carried on a satellite, an aircraft or the like. When a circuit element is highly integrated and the package is reduced in size and weight, the circuit carried on the package is damaged by heat generation since the ferrous material has a low thermal conductivity of 0.04 cal/.degree.C.cm.sec.
In order to solve such problems, the use of an aluminum package has been recently studied. However, although a desired value of thermal conductivity can be obtained in a package material made of a general aluminum alloy according to Japanese Industrial Standards (JIS) or ASTM, such a material has a high thermal expansion coefficient of at least about 20.times.10.sup.-6 /.degree.C., a value which is extremely different from that of a material forming a circuit substrate. When the circuit substrate is directly mounted on the surface of the package by Au--Sn solder or the like, the circuit substrate cracks due to thermal stress caused by the difference in thermal expansion coefficients. In order to solve this problem, it has been necessary to insert a member forming stress relaxation structure between the package and the circuit substrate. As the result, the overall structure of the package has been complicated.
It may be considered to reduce the thermal expansion coefficient of the aluminum alloy by adding at least 30 percent by weight, for example, of silicon to aluminum so that the content of silicon exceeds that of a conventional aluminum alloy according to JIS or ASTM. When such an Al--Si alloy is obtained by general casting, however, its machinability is extremely reduced since the particle size of primary silicon crystals is increased. Thus, it is difficult to precisely machine the alloy into a desired configuration required for a package.
Alternatively, the thermal expansion coefficient may be reduced by adding ceramic particles of silicon carbide or the like to aluminum. Also in this case, however, machinability of the alloy is substantially reduced.
When a package is manufactured, its members are suitably joined with each other by laser welding, to avoid a thermal effect on a microwave integrated circuit device which is carried within the case thereof. If a general aluminum alloy according to JIS is subjected to laser welding, however, cracking is caused following quenching since the welded zone is cooled at a higher rate than the cooling of the base metal. Thus, it is impossible to maintain an airtight package. In order to solve this problem, it has been proposed to insert an Al--Si brazing filler metal between the case and a lid member or to perform a nickel plating etc. for carrying out a laser welding step to solution-treat the nickel and silicon in the welding zone and to increase the heat resistance to prevent cracking as disclosed in Japanese Patent Laying-Open No. 58-223350(1983) or 60-31247(1985). Another method of utilizing a cast aluminum alloy having a high content of silicon as disclosed in Japanese Patent Laying-Open No. 58-223349(1983) also aims at solving the above problem. However, the former method requires an involved plating process or preparation of the brazing filler metal. In the latter method, the coarse primary silicon crystallizes as hereinabove described, which prevents a precise machining.
Further, a conventional aluminum alloy according to JIS has a lower Young's modulus of 7000 to 8000 kgf/mm.sup.2 than a ferrous material such as stainless steel. Therefore, it is necessary to increase the thickness of a case in order to guarantee the mechanical strength of a package formed by such a conventional aluminum alloy. Thus, the package cannot be effectively miniaturized.